While the Met Gala may raise money for an educational cause, much money is squandered on the excessive extravagance of the night. If less money was initially put towards the event (which is often used in wasteful and superficial ways), the event would have even more money by the end of the night.
Moreover, the money goes towards the Metropolitan Museum of Art's Costume Institute. While an educational cause, the millions of dollars earned via charity as well as the money spent on the night would certainly do more good put towards a more charitable cause. There is no need for a single exhibit to receive upwards of ten million dollars yearly. In this way, while the Met Gala certainly is a charity event, it is certainly not the most productive nor helpful.